This invention relates to light sensitive materials used to record small patterns with coherent radiation, and to methods of producing relief patterns. In particular, this invention relates to the recording of a phase relief hologram in a durable substrate which can be archivally stored or used as a master for replication by electroforming or embossing. This invention also relates to the recording of amplitude holograms. More generally, the present invention relates to recording of an information-bearing radiation field, and in particular to processes for such recording on a durable medium and wherein the radiation field has a characteristic feature size of below approximately one micron. Examples of such applications include the recording of holograms, semiconductor microlithography, and computer generated holography.
The general method for producing a phase relief hologram of the prior art consists of several steps. First, a photosensitive material is exposed to a light intensity field formed by the interference of two or more coherent beams of light. Often one of these beams is a reference source and one of these beams is modulated by an information signal. The photosensitive material is generally a photoresist, such as Shipley AZ1350J sold by the Shipley Company of Newton, Mass. This class of photosensitive materials are not very sensitive to light, and can require exposure times of 30 minutes or more using the 457.9 nm line of an 18-watt argon laser to expose a 6".times.6" hologram. After exposure, the surface of such a resist is developed to produce a surface relief pattern with a depth corresponding to the intensity of radiation at each point.
For mass production, it is generally desired to make a master hologram, which may be exposed using a laser source having a wavelength such as 514 nm (argon ion), a 633 nm (HeNe), a 647.1 nm (Kr ion) or other wavelengths such as produced by a ruby crystal laser. For such wavelengths, photographic emulsions such as Agfa Gevaert 8E56HD or 8E75HD, are generally used. When such an emulsion is used, however, 457.9 nm illumination cannot be used for "reading" the hologram to record it in photoresist, because distortion problems due to the wavelength shift between the hologram recording wavelength and reconstruction of the master hologram at 457 nm make this impractical. Often the master hologram is made in Agfa 8E56HD film using the 488 nm or 476 nm line of the argon laser. Use of these wavelengths minimizes the wavelength shift problem, but the silver halide emulsion still introduces scatter noise. Sometimes the master hologram is recorded directly onto photoresist. This produces a first generation, high quality hologram, but is expensive and requires large lasers.
Generally, holograms recorded in materials such as silver halide/gelatin emulsions, dichromated gelatin, or photopolymer are subject to Bragg selection problems when the recording and playback geometries differ. These problems arise due to the thickness of the recording material.